1. Field of the Invention
This invention relates to an electrode for an alkaline storage battery such as a nickel-metal hydride battery and nickel-cadmium battery, and more particularly to an improvement of the electrode capable of preventing removal of an active material.
2. Description of the Related Art
In recent years, in order to realize a high-energy alkaline storage battery, a nickel-metal hydride (Nixe2x80x94MH) battery using a hydrogen storage alloy electrode has attracted considerable attention and put into practice. Previously known hydrogen storage alloys used for the Nixe2x80x94MH battery are a Tixe2x80x94Ni alloy, La(or Mm (Misch metal: compound of a Ce group rare earth element))xe2x80x94Ni alloy, etc. The electrode using the hydrogen storage alloy can be manufactured by the process of adding a binding agent to the hydrogen storage-alloy to make an active material paste and filling an electrode plate of e.g. a punching metal with the active material paste thus acquired.
In this case, inclusion of the binding agent in the active material paste intends to increase the strength of the electrode, thereby preventing the active material from removed from the electrode and the surface of the electrode from cracking. Otherwise, application of the binding agent to the surface of the electrode filled with the active material paste intends to increase the strength of the electrode more greatly, thereby preventing the active material from removed from the electrode and the surface of the electrode from cracking.
Meanwhile, the binding agent to be added to the active material paste or applied to the electrode surface is an aqueous binder such as poly(ethylene oxide) (PEO) or polyvinylpyrrolidone (PVP), an aqueous emulsion elastomer such as styrene butadiene rubber (SBR), or phenol resin.
However, the binding capacity of these binders is low. This has presented a problem that the active material is removed during the process of manufacturing the electrode or removed within the electrode to shorten the cycle life.
The reason why such a phenomenon occurs is not clear. However, this is probably because the binding agent such as poly(ethylene oxide) (PEO) or polyvinylpyrrolidone (PVP) is dissolved due to water in the air and dissolved into an alkaline electrolyte within the battery so that the binding capacity between the active materials is reduced, thereby leading to removal of the active material. This is also probably because the binding capacity of the aqueous emulsion elastomer such as styrene butadiene rubber (SBR) having a rubber property is low so that the active materials are not bound to one another, thereby leading to removal of the active material. Further, this is probably because the phenol resin which is thermosetting does not have sufficient adhesion to the active materials so that the active materials are not sufficiently bound with one another, thereby leading to removal of the active material.
This invention has been accomplished in order to overcome the inconvenience as described above.
An object of this invention is to provide an alkaline storage battery having an excellent cycle life which is provided with an electrode free from removal of an active material, the electrode being made using a binding agent which is excellent adhesion to the active material and gives a high binding capacity of the active materials with one another.
In order to attain the above object, the electrode for the alkaline storage battery according to this invention includes a binding agent containing thermoplastic xylene-formaldehyde resin.
Since the thermoplastic xylene-formaldehyde resin is non-aqueous, it will be not dissolved into water in the air or the alkaline electrolyte within the battery. For this reason, the electrode using the thermoplastic xylene-formaldehyde resin as the binding agent can prevent removal of the active material during the manufacturing process of the battery or within the battery. Thus, an alkaline storage battery having an excellent cycle life can be obtained.
The thermoplastic xylene-formaldehyde resin softens up by the heat treatment in a drying step and thereafter solidifies in resin so that it can be given a high adhesion to the active material. This increases the binding capacity of the active materials to one another, thereby preventing removal of the active material. Accordingly, the alkaline storage battery with an excellent life can be obtained. Further, since the thermoplastic xylene-formaldehyde is thermoplastic, it softens up by the heat treatment to adhere to the active materials. Therefore, after having dried, it fixes the active materials to one another strongly. This prevents removal of the active material during the process of manufacturing the electrode which will be performed later and within the battery.
In this case, alkylphenyl-modified xylene-formaldehyde resin is non-soluble and thermoplastic, and has a property that it softens up by addition of heat and solidifies in resin as the temperature decreases. Therefore, the thermoplastic xylene-formaldehyde resin may be preferably the alkylphenyl-modified xylene-formaldehyde resin. Further, the particle of the active material having a larger particle diameter has a tendency of being easily removed from the electrode. Therefore, the thermoplastic xylene-formaldehyde resin having an excellent binding capacity can provide a sufficient effect of preventing removal of the active material for the electrode using the hydrogen storage alloy having a large average particle diameter.
Since the thermoplastic xylene-formaldehyde resin is non-soluble, a particular contrivance is required to manufacture the electrode using such a binding agent.
To this end, the method of manufacturing an alkaline storage battery according to this invention comprises the steps of: mixing an active material and an aqueous binder or aqueous emulsion binder to create active material slurry; applying or filling the active material slurry to or in an electrode substrate and drying the electrode substrate; and immersing the electrode substrate in a solution with thermoplastic xylene-formaldehyde resin dissolved in organic solvent and drying it.
Further, the method of manufacturing an alkaline storage battery according to this invention comprises the steps of: mixing an active material and an aqueous binder or aqueous agent to create active material slurry; applying or filling the active material slurry to or in an electrode substrate and drying the electrode substrate; emulsifying a solution of thermoplastic xylene-formaldehyde resin dissolved in an organic solvent to create an emulsion of the thermoplastic xylene-formaldehyde resin; and immersing the electrode substrate in the emulsion of the thermoplastic xylene-formaldehyde resin and drying it.
Further, the method of manufacturing an alkaline storage battery according to this invention comprises the steps of: emulsifying a solution of thermoplastic xylene-formaldehyde resin dissolved in an organic solvent to create an emulsion of the thermoplastic xylene-formaldehyde resin; mixing an active material and the emulsion of the thermoplastic xylene-formaldehyde resin to create active material slurry; and applying or filling the active material slurry to or in an electrode substrate and drying the electrode substrate.
In this way, the removal of the active material during the process of manufacturing the electrode or within the battery can be prevented by any of the techniques of immersing the active-material-applied or -filled electrode substrate in the solution of the thermoplastic xylene-formaldehyde resin dissolved; immersing it in the emulsion of the thermoplastic xylene-formaldehyde resin with an emulsifier; and applying or filling the active material slurry with the emulsion of the thermoplastic xylene-formaldehyde resin to or in the electrode substrate. Thus, the alkaline storage battery with an excellent cycle life can be obtained.
Incidentally, it should be noted that the active material preferably has a particle diameter within a range of 20-100 xcexcm.
Further, the thermoplastic xylene-formaldehyde resin may be preferably the alkylphenyl-modified xylene-formaldehyde resin.
Further, it should be noted that the active material is a hydrogen storage alloy capable of reversibly making electro-chemical absorption or desorption of hydrogen.
Further, it should be noted that a layer of the thermoplastic xylene-formaldehyde resin covers the outside of the active material layer.
Further, it should be noted that the thermoplastic xylene-formaldehyde resin is contained in the active material layer.
Further, it should be noted that the thermoplastic xylene-formaldehyde resin is contained in a boundary between the electrode substrate and active material layer.
The above and other objects and features of this invention will be apparent from the following description.
An explanation will be given the embodiment in the case where this invention has been applied to a hydrogen storage alloy electrode.
1. Creation of a Hydrogen Storage Alloy
The metallic elements of Mm (misch metal), Ni, Co, Al and Mn which are commercially available were weighed and mixed to provide a composition represented by MmNi3.4Co0.8Al0.2Mn0.6. The composite thus created was cast and dissolved in a high frequency dissolving furnace, and cooled to create an ingot of a hydrogen storage alloy. After the ingot of the hydrogen storage alloy had been heat-treated for 10 hours at 1000xc2x0 C., it was mechanically crushed in an atmosphere of nitrogen to provide an average particle diameter of 60 xcexcm so that a particle of the hydrogen storage alloy was created.
2. Preparation of a Binding Agent Solution
Alkylphenyl-modified xylene-formaldehyde resin (Nikanol HP-100 available from Mitsubishi Gas Chemical Co., Inc.) was dissolved in ethylcyclohexane to prepare a solution of the alkylphenyl-modified xylene-formaldehyde resin by 10% by mass. The solution of the alkylphenyl-modified xylene-formaldehyde was mixed with pure water and a surface active agent by 5% by mass (Tween 20 available from Atras Chem. Ind. Inc.) to prepare an emulsion of the alkylphenyl-modified xylene-formaldehyde resin by 10% by mass.
3. Creation of a Hydrogen Storage Alloy Electrode